1. Field of the Invention
The present invention relates to a projection display system for projecting an image, displayed on a miniature liquid crystal panel, onto a screen and a liquid crystal display device used in the projection display system.
2. Description of the Prior Art
A liquid crystal display device in general has light-weight and thin-structured features and, therefore, researches and development have largely been made on the liquid crystal display device. On the other hand, the liquid crystal display device having a relatively large image display area is generally difficult to manufacture. In view of this, a projection display system has aroused interests in the art. The protection display system is a product of compromise that provides a viewable picture on an enlarged scale by protecting a relatively small-sized image reproduced on a miniature display area of the liquid crystal display panel.
This type of projection display system currently available in the market employs a twisted nematic liquid crystal display device (hereinafter referred to as a TN LCD device) utilizing an optical rotation of liquid crystal material. This TN LCD device modulates light passing therethrough by varying the orientation of the liquid crystal material when a voltage is applied selectively to pixel electrodes. Polarizing plates are disposed respectively on front and rear sides of the TN LCD device with respect to the direction of travel of light towards the TN LCD device, having their polarizing axes laid perpendicular to each other. The polarizing plate on the front side of the TN LCD device (hereinafter referred to as a polarizer) has its polarizing axis laid either parallel or perpendicular to the axis of orientation of one of the substrates of the TN LCD device through which rays of light enter the TN LCD device. An orientation process is carried out at an angle of 45 degrees relative to signal lines formed in the TN LCD device. In general, the TN LCD device is operated under a mode (NW mode) in which a black display is effected when a voltage is applied.
The liquid crystal panel operating under the NW mode has a problem in that light tends to leak from the perimeter of each pixel. This problem occurs because the liquid crystal molecules are oriented both in a normal direction, in which they ought to be oriented, and a direction counter to the normal direction. This type of orientation is referred to as a reverse-tilt domain. This domain is produced when the direction in which the liquid crystal molecules set up by an electric field developed between the pixel electrodes and the signal lines is partially reverse. A portion of the liquid crystal molecules set up in a reversed direction pass through the polarizing plate (hereinafter referred to as an analyzer) disposed on the rear or exit side of the liquid crystal display panel regardless of the application of the voltage. In other words, light leak takes place. No light leak occur if they are oriented in the regular direction.
To eliminate this problem, an attempt has been made to remove some of the pixel electrodes which are located at a site where light leak occurs. However, this attempt is problematic in that, if some of the pixel electrodes are removed, the surface area at which light modulation takes place will be reduced accompanied by a reduction in amount of light transmitted through the liquid crystal panel and, in turn, by a reduction in brightness of the image displayed.
Another attempt is suggested to employ a light shielding layer (hereinafter referred to as a black matrix) of an increased width formed on a counterelectrode. It is, however, found that this alternative attempt has a problem similar to that discussed in connection with the first mentioned attempt.
On the other hand, the conventionally utilized projection display systems in which the TN LCD device is utilized as a light valve fall into two categories: a front model in which a projector and a screen are separate from each other, and a rear model in which, while a transmissive screen is fitted over the entirety of a cabinet, all optical elements are accommodated within the cabinet. Both models have aroused interests in the art since they can be made compact in size.
In these projection display systems, the higher the higher display contrast, the higher the image quality. However, so long as the light leak occurs, there is no way to increase the display contrast.